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Processes Affecting SeawaterComposition

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Is the ocean becoming saltier?

Salinity in the ocean is in a steady-statecondition because amount of salt added (inputfrom sources) equals the amount removed(output to sinks)

Ocean is in approx. chemical equilibrium;proportion and amounts of dissolved solidsremain constant.Known as the “steady state ocean”

Steady State Ocean

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Average length of time that an ion orelement remains in solution in the ocean

Ions with long residence times tend toaccumulate in the sea; those with shortresidence times are removed.

Rapid mixing and long residence timesexplain constant composition of seawater.

Residence Time

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Residence time can be calculated by theequation:

Residence Time = Conc. of element in the ocean Rate of addition or removal

The residence time of any element depends onits chemical activity.

Residence Time

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Conservative constituents of seawater occur inconstant proportions.

Conservative elements have long residence timesand are the most abundant dissolved salts in theocean.

Nonconservative constituents have short residencetimes, and are usually associated with seasonal,biological or short geological cycles.

Conservative andNonconservative Constituents

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Residence Time

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Dissolved Gases

Affected by biological activity

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Solubility and Saturation Value ofGases

Both increase as T and S decrease andpressure increases

Solubility - tendency to dissolve and gointo solution

Saturation value - equilibrium amount ofgas dissolved in water at an existingtemperature, salinity and pressure

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Concentration of dissolved gases:

Increases with decreasing T (cold water holds more dissolved gas)

Increases with decreasing SIncreases with increasing P

Dissolved Gases: Requiredfor Life Processes

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Undersaturation - contains less thanmaximum amount of dissolved gas

Saturation - maximum amount of gasSupersaturation - contains more gas

than saturation value (excess gascomes out of solution)

Surface layer - usually saturated due to gasexchange with the atmosphere

Below surface layer -gas content reflectsrespiration, photosynthesis, decay and inputfrom volcanic vents

Gases in Seawater

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Oxygen Content

Solubility depends on T and Salinity Warm, saline water holds less O2

Oxygen consumption in water column Respiration by animals

Mixing rate of oceans Sluggish mixing - low rate of O2 replenishment

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Oxygen Concentration WithDepth

Surface layer is rich in oxygen because ofphotosynthesis and diffusion from atmosphere

Oxygen minimum layer occurs at about 150 to1500m below the surface and coincides withthe pycnocline.

Sinking food particles settle into this layerand are slowed down by the densitygradient.

The food draws large numbers oforganisms which consume oxygen.

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Carbon Dioxide Concentration

Solubility depends on T and S

CO2 is released by animals

Mixing rate of oceans

Affects pH of seawater

Similar to concentration profiles ofnutrients (PO4

-3, NO3-)

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Carbon Dioxide Controls Acidity ofSeawater

Major sources - respiration and decayMajor sinks - photosynthesis and

construction of carbonate shells pH measures how acid or base water is.

• - pH of 0 to 7 is acidic.• - pH of 7 is neutral.• - pH of 7 to 14 is basic.

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pH is related to amount of CO2 dissolvedin water; combines with water toproduce carbonic acid which releasesH+ ions.

• CO2 + H2O ←→ H2CO3 ←→ H+ + HCO3-

←→ H+ + CO3-2

H2CO3 is carbonic acid, HCO3- is the

bicarbonate ion and CO3-2 is the

carbonate ion.

Gases in Seawater

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Changing the amount of CO2 shifts thereaction to the right or left of the equation. Adding CO2 shifts the reaction to the right

and produces more H+ ions making the watermore acid.

Removing CO2 shifts the reaction to the left,combining H+ ions with carbonate andbicarbonate ions and decreasing the acidity.

Gases in Seawater

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An acid is a substance that releases ahydrogen ion in solution.

A base is a substance that combines with a hydrogen ion in solution.

A solution containing a base is called analkaline solution.

Acidity or alkalinity is measured on the pHscale.

Acid-Base Balance

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Dissolved CO2 - buffers against largeshifts in pH

Carbonate shells dissolve in deep water;cold water under pressure holds more CO2

making the water more acidic (pH = 7.8)

Warm, shallow water contains lessdissolved CO2 and is less acidic than thedeep water; carbonate sediments do notdissolve (pH = 8.2)

Gases in Seawater

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19Note that the pH of seawater varies slightly with depth.

pH of Seawater

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Oxygenand CO2

Profilesare

Opposite

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Decay of organic matter consumesoxygen

Deep water has more oxygen because itis forms from cold surface waters sinkingto the bottom; oxygen usage is low die tofewer organisms in the deep water

Anoxic waters contain no oxygen and areinhabited by anaerobic organisms(bacteria).

Dissolved Oxygen in Seawater

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Absence of Oxygen in StagnantBasins

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Nutrients: Essential Chemicals for Life

Major nutrients are compounds ofnitrogen, phosphorus and silicon

Because of usage, concs. are very lowat the surface - parts per million (ppm)

Concs. vary greatly over time andspace; a nonconservative property ofseawater

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Nutrient,Oxygen and

pH Profiles inTypical

Seawater

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Total Dissolved Inorganic Carbon Profiles

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Nutrient Profiles

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Describes exchange of water amongocean, land and atmosphere

Reservoirs are: Oceans - contain 97% of Earth’s water

Rivers, lakes and glaciers

Groundwater - larger volume than all of thecombined water in lakes and rivers

The Hydrologic Cycle

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The Hydrologic Cycle

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Desalinization of Seawater

Produces potable water by: Distillation - evaporation of seawater and

condensation of vapor

Freezing - produces salt-free ice to bemelted for water

Reverse osmosis - seawater underpressure is forced through a semi-permeable membrane

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Many processes regulate the composition of seawater.The concept of a steady state ocean summarizes the ideathat ions are added to and removed from the ocean at thesame rate.

Summary